(a) Field of the Invention
This invention relates to drive systems for transmitting torque at the crankshaft of an automotive engine to engine accessories including a supercharger, a compressor for an automotive air conditioner, and a generator, and more particularly relates to measures for striking a balance between the acceleration and fuel consumption performances of an automotive engine.
(b) Description of the Related Art
An example of known drive systems for driving a supercharger using torque at an engine crankshaft is a continuous engagement type one in which as schematically shown in FIG. 10, a ribbed belt 204 is entrained between a crank pulley (not shown) as an input member coupled to the engine crankshaft and a ribbed pulley 203 as an output member coupled to a rotational shaft 202 of a supercharger 201. In this system, torque at the crankshaft is transmitted via the ribbed belt 204 to the rotational shaft 202 to rotate the rotational shaft 202 and thereby drive the supercharger 201.
Superchargers generally provide an amount of boost predetermined for full engine load conditions. Therefore, in the supercharger 201 of the continuous engagement type drive system, as shown in FIG. 11, the ratio of the rpm of the rotational shaft (supercharger rpm) to the rpm of the crankshaft (engine rpm), i.e., the speed ratio, is constant (i.e., equal to (the crank pulley diameter)/(ribbed pulley diameter)). This means that at low engine load, the amount of boost becomes excessive and the excess provides a loss of power to drive the supercharger 201, leading to deteriorated fuel economy. In particular, under normal driving conditions, the engine is less likely to fall into full load conditions and therefore the fuel economy problem is outstanding.
As a technique to cope with this problem, there is known one as disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 9-13979). In such a technique, an on/off type solenoid-operated clutch is placed in the torque transmission path leading from the crankshaft to the rotational shaft of the supercharger so that at low engine load, torque transmission to the supercharger is shut off to prevent the startup of the supercharger.
Alternatively, there is also known a technique as disclosed in Patent Document 2 (Japanese Unexamined Patent Publication No. 6-257461), in which two solenoid-operated clutches, one for low speed ratio and the other for high speed ratio, are arranged in parallel in the torque transmission path. With this technique, at low engine load, the clutch for low speed ratio is turned on to keep the rpm of the rotational shaft low. Only when engine torque is necessary, the clutch for high speed ratio is turned on to make the rpm of the rotational shaft higher.
Accessories such as a compressor for an automotive air conditioner and a generator, unlike the supercharger, are needed to exhibit a certain level of ability even at low engine rpm, for example, at idling speed. On the other hand, when the engine rpm becomes high during actual cruise, such accessories are driven at an rpm higher than required (waste the drive power) thereby deteriorating fuel economy. Therefore, as also described in the above-mentioned Patent Document 2, it is known that the accessories are driven by selectively using the two solenoid-operated clutches for high speed ratio and low speed ratio.
The above techniques, however, have a problem of increasing the weight, cost and power consumption of the drive system by an added solenoid-operated clutch or clutches.